UC Berkeley

In order to fully understand the processes that influence phenotypic evolution, it is necessary to explore the role that developmental channeling plays in shaping morphology. In my doctoral research, I have investigated how an increased understanding of ontogenetic patterns gives insight into the evolution of birds. Here, I present the results of an integrative study combining material from multiple levels of the biological hierarchy (clade, individual organism, and cell), using both paleontological and neontological data. Specifically, I studied post-natal gross morphological change and cellular growth in a phylogenetically broad sampling of Cretaceous and modern avians, using ontogenetic series when available.

A three-dimensional geometric morphometric analysis of ontogenetic cranial shape change indicates that a relatively high level of developmental constraint governs the evolution of the skull, although shape at onset of growth is a more variable characteristic, in large part because hatchling chicks already have cranial morphology very similar to that of adults. Histological study of extant bird skeletons also indicates high variability in onset of growth among taxa and reveals a common pattern of greater skeletal maturity of the femur relative to the humerus at the time of hatching. This is interpreted as an example of a ‘spandrel,’ later exapted by semi-precocial taxa in which pelvic limbs appear to achieve functional maturity before pectoral limbs for adaptive reasons. Histological analysis of Cretaceous avians indicates this trait of differential maturity between limb elements may have a very deep evolutionary origin, and could explain substantial microstructural differences that persist into adulthood in some extinct taxa. I focused in particular on growth in members of the Enantiornithes, and further conclude that this group exhibits a greater diversity of growth strategies than originally hypothesized, although with no apparent relationship to body size or geological age, and that this clade had a unique developmental mode not directly comparable that of to any crown-group birds. Finally, phylogenetic and functional constraints appear to have a much greater influence on cellular growth of long bones than on cranial shape change, for which developmental channeling is a primary influence.